Document ZFS_DKMS_ENABLE_DEBUGINFO in userland configuration

Document the ZFS_DKMS_ENABLE_DEBUGINFO option in the userland
configuration file, as done with the other ZFS_DKMS_* options.

It has been introduced with commit e45c173 ("dkms: Enable
debuginfo option to be set with zfs sysconfig file") but isn't
mentioned anywhere other than the 'dkms.conf' file (generated).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Closes #9191

etc/init.d/zfs.in

@@ -91,6 +91,10 @@ MOUNT_EXTRA_OPTIONS=""
 # Only applicable for Debian GNU/Linux {dkms,initramfs}.
 ZFS_DKMS_ENABLE_DEBUG='no'
 
+# Build kernel modules with the --enable-debuginfo switch?
+# Only applicable for Debian GNU/Linux {dkms,initramfs}.
+ZFS_DKMS_ENABLE_DEBUGINFO='no'
+
 # Keep debugging symbols in kernel modules?
 # Only applicable for Debian GNU/Linux {dkms,initramfs}.
 ZFS_DKMS_DISABLE_STRIP='no'